1. Field of the Invention
The present invention relates to a semiconductor laser apparatus which detects the laser beam from a semiconductor laser element and uses a light-receiving element so as to perform automatic light power control.
2. Description of the Prior Art
FIG. 4 shows a semiconductor laser apparatus for automatically performing light power control. A semiconductor laser element 11 of the semiconductor laser apparatus has a first output end face 12 which emits output light and a second output end face 13 which emits light that is to be monitored.
An end face protection film 14 formed of an Al.sub.2 O.sub.3 film is formed on the first output end face 12, and an end face protection film 15 consisting of an Al.sub.2 O.sub.3 film 15a and an amorphous Si film 15b is formed on the second output end face 13.
A monitoring light-receiving element 16 is obliquely arranged with respect to the semiconductor laser element 11 at the side of the second output end face 13 of the element 11. A surface protective film 17 is formed on the light-receiving surface of the light-receiving element 16.
The reason why the light-receiving element 16 is obliquely arranged is to prevent the monitored light which reflected by the light-receiving element 16 from returning to the semiconductor laser element 11.
The wavelength of the laser beam emitted from the semiconductor laser element 11 has a temperature dependency of about 2.5 .ANG./deg, as shown in FIG. 5. When the light-receiving element 16 is obliquely arranged as described above, the reflectivity of the surface protection film 17 varies and depends on the wavelength of incident light. Therefore, the ability of the light-receiving element 16 to detect power also has a temperature dependency.
The temperature dependency of the detected power of the light-receiving element 16 may be positive or negative with respect to a change in temperature and depends on the characteristics of the individual light-receiving elements 16.
Assuming that the temperature dependency of the detected power of the light-receiving element 16 is positive with respect to a change in temperature, even if the power of the semiconductor laser element 11 is constant, the detected power of the light-receiving element 16 increases as the temperature increases, as shown by a curve 21 in FIG. 3.
The reflectivities of the end face protection films 14 and 15 of the semiconductor laser element 11 change according to the thickness of the films. FIG. 1 shows the relationship between the thickness of the Al.sub.2 O.sub.3 film and the reflectivity. Note that the reflectivity when the thickness is 0 is equal to that of GaAs from which the semiconductor laser element 11 is formed.
As can be seen from FIG. 1, for example, the reflectivity shown by curve 22 becomes minimum when the film thickness is about 1,200 .ANG., and has an identical value for two different film thicknesses. In a conventional apparatus, in order to obtain a desired reflectivity, the thickness of the Al.sub.2 O.sub.3 film is set to be less than 1,200 .ANG..
The reflectivity of Al.sub.2 O.sub.3 also changes in accordance with the wavelength of incident light. The reflectivity 22 corresponds to incident light having a wavelength of 790 nm. A reflectivity curve 23 corresponds to incident light having a wavelength of 800 nm. For this reason, the power of the semiconductor laser element 11 on which the end face protection films 14 and 15 are formed also has a temperature dependency.
Since the end face protection film 15 includes the amorphous Si film 15b in addition to the Al.sub.2 O.sub.3 film 15a in order to increase the ratio of a laser beam emitted from the first output end face 12, and hence obtain a higher reflectivity than that of the end face protection film 14, the film 15 has a smaller temperature dependency than the end face protection film 14.
Therefore, if a current is supplied to the semiconductor laser element 11 to obtain constant power output light which is emitted from the first output end face 12, the power of the monitored light emitted from the second output end face 13 has a temperature dependency with respect to the film thickness of the end face protection film 14, as shown in FIG. 2.
Since the thickness of the Al.sub.2 O.sub.3 film constituting the end face protection film 14 is smaller than 1,200 .ANG., as described above, the temperature dependency of the power of the semiconductor laser element 11 is positive with respect to a change in temperature, as can be seen from FIG. 2.
As a result, the temperature dependency of the power of the semiconductor laser element 11 is superposed on that of the curve 21 of the light-receiving element 16 as shown in FIG. 3. Therefore, as shown in FIG. 3, a curve 24 of the semiconductor laser apparatus has a very large temperature dependency.